Method for producing decomposer of organic halogenated compounds

ABSTRACT

A method for producing a decomposer of an organic halogenated compound comprises subjecting an iron powder produced beforehand to plastic deformation that gives the iron powder particles a flat shape. Further, an iron powder and a copper salt powder are mechanically mixed in a ball mill to produce a copper salt-containing iron particle powder in which the particles of the two powders are joined. In this case, the method for producing the decomposer of an organic halogenated compound is characterized in that the iron powder is mechanically deformed to give the particles a flat shape.

TECHNICAL FIELD

This invention relates to a method for producing a decomposer fordecomposing organic halogenated compounds contained in soil, undergroundwater and the like.

BACKGROUND ART

Recent years have seen advances in the development of technologies forpurifying soil, groundwater and the like contaminated with organichalogenated compounds, by decomposing the compounds so as to reduce thecontamination level. This development is directed basically to obtainingdecomposers with high capability to decompose the organic halogenatedcompounds constituting the contaminants. Iron powder is a typicaldecomposer. For example, Patent Document 1 teaches that soilcontaminated with an organic halogenated compound like trichloroethylenecan be effectively treated to decompose the trichloroethylene or thelike in the soil by mixing therein iron powder containing 0.1 wt % ormore of C (carbon) and having a specific surface area of 500 cm²/g orgreater.

Patent Document 2 teaches that high-purity iron powder containing C:less than 0.1 mass %, Si: less than 0.25 mass %, Mn: less than 0.60 mass%, P: less than 0.03 mass %, S: less than 0.03 mass % and O: less than0.5 mass % is effective for purifying soil or water contaminated with arecalcitrant organic halogenated compound like 1,2-cis-dichloroethylene(cis-1,2-DCE).

However, a need was felt for a decomposer with even strongerdecomposition capability. To meet this need, Patent Document 3, PatentDocument 4 and Patent Document 5, which are in the name of the sameapplicant as the applicant of this application, proposecopper-containing iron powders obtained by depositing metallic copper onthe surfaces of iron powder particles. When such a copper-containingiron powder is added to and mixed with soil, groundwater or the likecontaminated with an organic halogenated compound, the organichalogenated compound is efficiently decomposed. Nevertheless, adecomposer with even higher decomposition is desired.

Patent Document 1: JP 11-235577A

Patent Document 2: JP 2002-316050A

Patent Document 3: JP 2000-005740A

Patent Document 4: JP 2002-069425A

Patent Document 5: JP 2003-339902A

PROBLEM TO BE OVERCOME BY THE INVENTION

The problem to be solved by the present invention is to efficientlyobtain at low cost a decomposer that exhibits still higher decompositioncapability than heretofore even with respect to recalcitrant cis-1,2-DCEand other organic halogenated compounds.

MEANS FOR SOLVING THE PROBLEM

The inventors discovered that the capacity of an iron powder todecompose organic halogenated compounds is enhanced by mechanicallymodifying the particle shape of an iron powder produced beforehand so asto modify its surface condition and also reform the structure of thepowder interior. More specifically, it was found that when iron powderparticles are crushed and spread out by plastic deformation, the inneriron matrix of the particles comes to be exposed at the surface and theparticles are deformed into a flattened shape by the spreading, and thatthis works to increase the decomposition performance of the iron powderwith respect to organic halogenated compounds. The present inventiontherefore provides a method for producing a decomposer of organichalogenated compounds that comprises subjecting iron powder particlesproduced beforehand to plastic deformation that gives the iron powderparticles a flat shape. The plastic deformation of the iron powderparticles can be conducted using a ball mill, particularly a vibratingball mill. The flat particles preferably have a planar ratio of 2 orgreater.

It was further found that an organic halogenated compound decomposerwell suited for decomposition of recalcitrant cis-1,2-DCE and the likecan be obtained by mechanically mixing an iron powder and a copper saltpowder to obtain a copper salt-containing iron particle powder havingcopper salt physically joined to the iron powder particles. It was foundthat in this case an organic halogenated compound decomposer even bettersuited for decomposition of recalcitrant cis-1,2-DCE and the like can beobtained by loading the iron powder and the copper salt powder into aball mill and conducting plastic deformation that that gives the ironpowder particles a flat shape and physically joins the copper saltpowder to the surface of the iron powder particles. The presentinvention therefore provides a method for producing a decomposer oforganic halogenated compounds characterized in that, at the time ofproducing a copper salt-containing iron particle powder by mechanicallymixing an iron powder and a copper salt powder in a mill (particularlyin a vibrating ball mill) to join the particles of the two powders, themechanical mixing plastically deforms the powders to give the particlesa flat shape. The flat particles preferably have a planar ratio of 2 orgreater.

The mechanical mixing is best conducted by the dry method and the ironpowder is preferably reduced iron powder. Further, in order to avoiddissolution by the water of crystallization of the copper salt powderduring mixing with the iron powder, the copper salt powder used isdesirably one that has been dehydrated beforehand to remove part or allof the water of crystallization.

In accordance with the present invention, it is possible to obtain adecomposer having high decomposition capability with respect to evenrecalcitrant organic halogenated compounds such as cis-1,2-DCE,efficiently and at low cost, and to make a major contribution to thepurification of, for example, soil, groundwater and other kinds of watercontaminated with such organic halogenated compounds.

PREFERRED EMBODIMENTS OF THE INVENTION

In the present invention, the organic halogenated compounds to bedecomposed include so-called VOCs, and, for example includedichloromethane, carbon tetrachloride, 1,2-dichloroethane, 1,1-DCE,cis-1,2-DCE, 1,1,1-trichloroethane, 1,1,2-trichloroethane, TCE,tetrachloroethylene, 1,3-dichloropropene, trans-1,2-dichloroethylene,trihalomethane, PCB, dioxin and, the like. These organic halogenatedcompounds may be present individually or in combinations. The decomposerof the present invention exhibits a catalytic effect in thedehalogenation reaction of an organic halogenated compound. Althoughfluorine and chlorine are the typical halogen elements, the decomposerof the present invention is particularly suitable for decomposition oforganic chlorinated compounds.

The decomposer of the present invention is for treating water, soil,inorganic substances, composites thereof and the like contaminated withorganic halogenated compounds such as the foregoing and is capable ofdecomposing these organic halogenated compounds. It is usefulparticularly in environmental applications for purification ofwastewater, groundwater, soil, exhaust gases and the like contaminatedwith the aforesaid organic halogenated compounds.

When the decomposer of the present invention is used for purification ofwastewater, groundwater, soil, exhaust gases and the like contaminatedwith the aforesaid organic halogenated compounds, implementation ispossible, for example, using an earth auger or other large civilengineering machinery and equipment conventionally employed inpurification operations, while flexible containers, paper bags and othercommercially available packing containers are sufficient for storage ofthe decomposer. The decomposer according to the present invention istherefore excellent in both handling and storage property.

In the production of the decomposer according to the present invention,the starting iron powder used can, for example, be atomized iron powdermanufactured by atomizing molten iron or reduced iron powder produced byreducing iron ore. The particle diameter of the starting iron powder isnot particularly limited. Although the starting iron powder need onlyhave iron as its main constituent, it should preferably not containchromium, lead or other constituents that may be a source of secondarycontamination. A copper salt powder is preferable as the copper sourcestarting material joined to the iron powder, and copper sulfate, forexample, can be used as the copper salt powder. It is possible to usecopper oxide or metallic copper instead of copper salt.

Production of the decomposer of the present invention requires primarilythe particles of the starting iron powder to be flattened by impartingplastic deformation thereto, and concretely is essentially a process ofcrushing and flattening the iron powder particles. For example, the ironpowder starting material is loaded in a mill and the particles areflattened. The mill used is preferably a vibrating ball mill of the typethat vibrates a case into which many hard balls measuring several mm indiameter are charged. When the case is vibrated, the balls insidevibrate and collide, so that when starting iron powder is present in thecase, the particles of the iron powder are crushed and spread out. Ironpowder formed into the desired flattened shape can be obtained byregulating the vibration period and amplitude, number of balls charged,amount of starting material charged, and the atmosphere. When a suitableamount copper salt powder is made co-present at this time, a flat ironpowder containing copper salt can be obtained. Slippage between theballs and the powders should preferably be suppressed so that the ballmill can effectively conduct plastic deformation, and therefore, it isadvisable in this invention to avoid use of dispersants and lubricantsgenerally utilized in ball mills to facilitate mixing and pulverization.

Copper sulfate powder is preferable as the copper salt powder. Althoughcopper sulfate is ordinarily procured in the form of CuSO₄.5H₂Oincluding water of crystallization, the water of crystallization shoulddesirably be removed as much as possible when used in the inventionmethod for producing a decomposer. Moisture from water ofcrystallization, moisture adhering to the mill surface, moisture in theatmosphere and moisture from elsewhere may produce an aqueous solutionof copper sulfate during mixing of the iron powder and copper sulfate,in which case Cu ions in the aqueous solution may deposit on the ironparticle surfaces as reduced metallic copper so that the iron particlesurfaces are liable to become coated with a film of the depositedmetallic copper. If the iron powder particle surfaces should becompletely covered with metallic copper, the performance of thedecomposer may be degraded. The water of crystallization of the coppersulfate is therefore preferably removed to the utmost possible, themixing with the iron powder is preferably conducted by a dry method thatminimizes moisture entrainment, and the mixing is preferably carried outin an inert gas atmosphere. CuSO₄.5H₂O can be removed of water ofcrystallization by heating. For example, 2 water molecules can beremoved by 45° C. heating, 4 by 110° C. heating, and all 5 by 250° C.heating.

The decomposer according to the present invention comprises ironparticles of a flat shape having a planar ratio of 2 or greater. If theplanar ratio is less than 2, the decomposition reaction velocityconstant κobs (abbreviated to κ) discussed later does not reach 0.2 andthe organic halogenated compound decomposition capability is low. On theother hand, the value of κ saturates when the planar ratio exceeds 15.The planar ratio is therefore preferably in the range of 2 to 15. Theplanar ratio is the ratio of flat surface diameter to thickness of theindividual particles measured with an electron microscope (SEM). Themeasurement is done by randomly selecting 50 particles from within theSEM image and averaging their planar ratios. When the measurement isbased on an SEM image photograph, it is advisable to enlarge thephotograph to make the largest diameter of one of the particle around 10mm. More specifically, it is possible to make the SEM observation at amagnification of 100-150× and directly measure the image using the scaleof a digital caliper or the like.

In this case, the average diameter of the flat particles is calculatedas follows. Fifty powder particles within the field of view are measuredin the direction of the flat surface for their major axis length andminor axis length perpendicular thereto, the flat surface diameters arecalculated as (major axis length+minor axis length)/2, and the averageflat surface diameter of the 50 particles is calculated. Further, thethickness of the particles is measured and the average thickness of the50 particles calculated. The average flat particle diameter is thencalculated by the following equationAverage flat particle diameter=(Average flat surface diameter×2+Averagethickness)/3.

The planar ratio is calculated by the following equationPlanar ratio=Average flat surface diameter/Average thickness.

The average diameter of the flat particles according to the presentinvention is 1-500 μm, preferably 25-250 μm. Typically, the average flatsurface diameter is in the range of 50-500 μm, the average thickness isin the range of 1-50 μm, and the planar ratio is desirably 2-15. Owingto this flat shape, excellent decomposition capability can bemanifested, but excessive flatness degrades the fluidity of the powder,making it inconvenient to handle, and, moreover, no further improvementin decomposition performance can be expected, so that the flat shape andparticle diameter set out above is desirable.

The particles of the decomposer in accordance with the present inventionare larger in surface area than those of the starting iron powder priorto plastic deformation, and owing to the increase in surface area by theplastic deformation, new surface areas are formed where the internalstructure of the particles is exposed at the surface, whereby thedecomposition performance of the iron powder with respect to organichalogenated compounds is enhanced. The reason for this is not clear butit is thought that there is collective involvement of different factors,such as that the number of reaction sites is increased by the formationof new surface areas at the particle surface, that the specific surfacearea of the iron powder increases, and that the surface condition of theparticles is modified so as to improve the adherability of volatileorganic compounds (VOCs) and further improve water wettability, therebyimproving contactability with contaminants. Moreover, the presence ofcopper, sulfur and acid radicals at or near the iron powder surfacefurther increases decomposition performance.

In the Examples set out below, the decomposition capability of theorganic halogenated compound decomposer is rated by using the testmethod in accordance with the following procedures a) to f) to calculatethe decomposition reaction velocity constant κobs (denoted simply as κ)with respect to cis-1,2-DCE.

a) Place 10 g kaolin and 0.1 g decomposer in a 20 ml vial and mixuniformly.

b) Add 6 g ion exchanged water to the foregoing mixed powder, allow itto thoroughly acclimatize, and enable hermetic sealing by using analuminum cap to tighten a butyl rubber stopper having a fluororesinliner.

c) Using a microsyringe, further inject 1 μL each of cis-1,2-DCE andbenzene, then hermetically seal.

d) Allow to stand in a 25° C. thermostatic bath for 1 hour, sample 0.1mL of the headspace gas, measure the gas concentration of thecis-1,2-DCE injected in c) with a gas chromatograph, and define thisconcentration as the initial value C₀.

e) At regular intervals over a 4-day period following d), analyze theheadspace gas and measure the cis-1,2-DCE gas concentration C with thegas chromatograph to evaluate the cis-1,2-DCE concentration attenuation.

f) Express the cis-1,2-DCE decomposition reaction velocity constant κ(unit: day⁻¹) using the following equation (where t represents dayselapsed from the time of the initial value)ln(C/C ₀)=−k×t.

WORKING EXAMPLES Example 1

As the starting powder was used a reduced iron powder comprising S:0.012%, C: 0.26%, O: 1.61% and the balance substantially of iron, whichpowder had an average particle diameter of 100 μm, an apparent densityof 2.7 g/cm³, and a BET specific surface area of 0.17 m²/g. This reducediron powder, 100 g, was charged into a vibrating ball mill together withan amount of copper sulfate (CuSO₄.5H₂O) powder such that the amount ofcopper was 1 mass % relative to the amount of iron of the reduced ironpowder (i.e., at the ratio of Cu/Fe=0.01). The vibrating ball mill wasfurther charged with 5 mm diameter zirconia balls whose volume was equalto 50 vol % of the internal volume of the mill. Next, the internalatmosphere of the mill was replaced with nitrogen gas and the mill wasoperated for 4 hours at a vibration frequency of 1250 vpm and amplitudeof 9 mm to mechanically mix the reduced iron powder and copper sulfate.After the mixing was stopped, the powder inside was removed into theatmosphere.

Measurement of the powder obtained with a laser diffraction particlesize analyzer showed it to have an accumulative particle sizedistribution in mass: D10 of 14.6 μm, D50 of 58.8 μm, and D90 of 121.8μm. The planar ratio was 8.8, the BET specific surface area was 1.03m²/g, and the powder copper content was 0.83 mass %. The powder wassubjected to the aforesaid organic halogenated compound decompositioncapability evaluation test. The cis-1,2-DCE concentration on day fourwas 99% down from the initial value, with only 1% remaining, and thedecomposition reaction velocity constant κ was 1.2 day-1.

Examples 2-8

Example 1 was repeated except that the kind of copper sulfate used asthe copper salt powder (each kind of copper sulfate differing in waterof crystallization content), iron powder and copper sulfate mixing ratio(Cu/Fe mass ratio), kind of substitutional gas in the mill, and milloperating conditions (vibration frequency, amplitude, time) weremodified to the conditions of the respective Examples (Examples 2-8)shown in Table 1. The powders obtained in the Examples were evaluated bythe method used in Example 1 and the results are also included in Table1.

TABLE 1 Product Production conditions Decom- Operating conditionsposition Vibration perfor- Fe starting Cu starting Cu addition frequencyAmplitude Time BET mance Planar powder powder Cu/Fe method AtmosphereVPM mm Hr m²/g κ ratio Example 1 Reduced CuSO₄•5H₂O 0.01 Dry N₂ 1250 9 41.03 1.2 8.8 Example 2 Reduced CuSO₄•5H₂O 0.01 Dry N₂ 1250 9 2 0.33 0.754.2 Example 3 Reduced CuSO₄•5H₂O 0.01 Dry N₂ 1250 9 1 0.36 0.45 3.2Example 4 Reduced CuSO₄•H₂O 0.01 Dry N₂ 1250 9 1 0.35 0.44 3.2 Example 5Reduced CuSO₄•H₂O 0.01 Dry Air 1250 9 1 0.33 0.54 3.8 Example 6 ReducedCuSO₄•5H₂O 0.01 Dry Air 1250 9 1 0.33 0.39 2.8 Example 7 ReducedCuSO₄•H₂O 0.02 Dry Air 1250 9 1 0.52 3.09 6.1 Example 8 ReducedCuSO₄•H₂O 0.03 Dry Air 1250 9 1 0.71 3.54 3.8 Example 9 Reduced 0 N₂1250 9 4 0.31 0.01 2.4 Comparative Reduced 0 0.03 <0.01 1.4 Example 1Comparative Spongy 0 3.1 <<0.01 1.5 Example 2 Comparative ReducedCuSO₄•H₂O 0.01 Dry N₂ 0.03 0.08 1.4 Example 3

The following can be seen from the results in Table 1.

In Examples 1-3, production was carried out under the same conditionsexcept that the run time was varied. It can be seen that the planarratio and κ value both increased with increasing run time.

In Examples 5, 7 and 8, production was carried out under the sameconditions except that the Cu/Fe mixing ratio was varied. It can be seenthat the κ value increased with higher Cu/Fe mixing ratio, namely, asthe ratio rose from 0.01 to 0.02 to 0.03.

In Examples 3 and 5, production was carried out under substantially thesame conditions except that copper sulfates having different amounts ofwater of crystallization were used. The κ value was higher in Example 5using copper sulfate monohydrate than in Example 3 using copper sulfatepentahydrate, indicating that use of a copper salt powder low in waterof crystallization improved decomposition capability.

In Examples 4 and 5, and in Examples 3 and 6, production was carried outunder the same conditions except that the atmospheric gas inside themill differed between nitrogen gas and air. Improved κ value was notedwhen the nitrogen gas atmosphere was used.

In the decomposition capability tests conducted on the powders obtainedin Examples 1-8, the same tests were repeated using 50 ml unaerated purewater instead of kaolin. At the first measurement conducted afterpassage of 1 hour, all cis-1,2-DCE gas densities C had fallen below theanalytical limit. From this result it was found that the decompositionvelocity of these decomposers is outstandingly high.

Example 9

Example 1 was repeated except that no copper sulfate was added. Thepowder obtained was evaluated by the method used in Example 1 and theresults are also included in Table 1.

Comparative Examples 1 and 2

For comparison, the BET value and κ value of the starting iron powderper se used in Example 1 are included in Table 1 as Comparative Example1 and the BET value and κ value of a commercially available spongy ironpowder per se are included therein as Comparative Example 2.

From the results in Table 1, it can be seen that the iron powderobtained in Example 9 had a larger planar ratio and a higher BET valuethan the starting iron powder of Comparative Example 1 and the spongyiron powder of Comparative Example 2, and was also improved in κ value.

Comparative Example 3

A powder was obtained simply by adding copper sulfate powder to reducediron powder, without processing the powders in a vibrating ball mill.The κ value of the powder was a low 0.08 day⁻¹.

Example 11

Example 1 was repeated at various Cu/Fe mixing ratios and vibrating ballmill run times. Powders with the particle size distributions and BETvalues shown in Table 2 were obtained. The κ values of the powders weremeasured and the results are shown in Table 2. The half-value widths inTable 2 are those of statistically processed particle size distributioncurves. A smaller half-value width means the spread of the particledistribution was narrower.

From the results in Table 2, it can be seen that by selecting the Cu/Femixing ratio and the degree of particle plastic deformation of thepowder obtained by the invention method, it is possible to obtain anorganic halogenated compound decomposer having a high decompositioncapability κ value.

Compo- Half- BET/ sition D10 D50 D90 value BET Particle Cu μm μm μmwidth m²/g diameter κ Remark 0.83 14.6 58.8 121.8 97.9 1.03 1.8E−02 1.20Example 1 0.90 31.6 75.5 135.2 92.6 0.33 4.4E−03 0.75 Example 2 0.9023.5 71.1 137.3 97.3 0.40 5.6E−03 0.48 0.83 25.3 77.4 150.2 107.6 0.364.7E−03 0.45 Example 3 0.61 22.6 73.4 148.8 114.7 0.35 4.8E−03 0.44Example 4 0.68 25.3 77.5 160.4 114.0 0.34 4.4E−03 0.34 0.81 24.6 76.5154.7 112.9 0.37 4.8E−03 0.31 0.84 24.8 76.2 152.9 108.0 0.35 4.6E−030.41 1.00 19.7 60.8 117.2 87.6 0.41 6.8E−03 0.55 0.89 23.1 65.0 120.986.5 0.38 5.8E−03 0.49 0.73 18.2 63.5 130.3 102.3 0.38 6.0E−03 0.46 0.6021.2 67.2 135.6 0.34 5.0E−03 0.32 0.67 25.7 72.5 143.4 108.3 0.324.4E−03 0.36 0.95 21.3 71.5 151.8 119.6 0.33 4.7E−03 0.39 Example 6 1.3218.5 66.6 147.4 112.2 0.41 6.2E−03 0.45 1.45 16.5 63.5 141.5 111.0 0.477.3E−03 0.46 1.62 8.1 44.6 115.8 120.7 0.66 1.5E−02 0.47

Example 12

This reduced iron powder, 100 g, was charged into a vibrating ball milltogether with an amount of copper sulfate (CuSO₄.5H₂O) powder such thatthe amount of copper was 1 mass % relative to the amount of iron of thereduced iron powder (i.e., at the ratio of Cu/Fe=0.01).

As the starting powder was used a reduced iron powder comprising S:0.012%, C: 0.26%, O: 1.61% and the balance substantially of iron, whichpowder had an average particle diameter of 100 μm, an apparent densityof 2.7 g/cm³, and a BET specific surface area of 0.17 m²/g. Using avibrating ball mill, this reduced iron powder together with coppersulfate (CuSO₄.5H₂O) was subjected to processing for flattening the ironpowder and processing for joining the iron powder and copper sulfate,using at an amount of the copper sulfate powder such that the amount ofcopper was 1 mass % relative to the amount of iron of the reduced ironpowder (i.e., at the ratio of Cu/Fe=0.01). The vibrating ball mill wascharged with 5 mm diameter zirconia balls whose volume was equal to 50vol % of the internal volume of the mill, the interior of the mill wascharged with an atmosphere of nitrogen gas during processing, and themill was operated at a vibration frequency of 1250 vpm and amplitude of9 mm to mechanically mix the reduced iron powder and copper sulfate.After the mixing was stopped, the powder inside was removed into theatmosphere.

In carrying out the processing, the rate of charging the total amount ofreduced iron powder and copper sulfate into the vibrating ball millduring operation was varied between 3 levels as in the following Tests1-3. Test 1: mill charging rate, 13.5 Kg/h. Test 2: mill charging rate,40.0 Kg/h. Test 3: mill charging rate, 400 Kg/h. In each test, the timefrom the start of charging to the completion of processing was 4 hours.The powders obtained in the tests were measured for average particlediameter, axis ratio (average major axis length/average minor axislength), planar ratio, and κ value. The results are shown in Table 3.For comparison, the measurement results for the mixed powder of reducediron powder and copper sulfate prior to charging into the vibrating ballmill are also shown in Table 3.

TABLE 3 Starting Mill Average iron Copper charging particle Axis Planarpowder content rate diameter ratio ratio κ value Test 1 Reduced 1% 13.5Kg/h   127 μm 1.64 8.8 1.2 Test 2 iron 40.0 Kg/h  88.6 μm 1.4 4.2 0.75Test 3 powder  400 Kg/h 102.8 μm 1.45 2.26 0.26 Comparison 1 Unprocessed103.8 μm 1.42 1.42 0.05

From the results shown in Table 3, it can be seen that the planar ratioincreased with decreasing charging rate and that the κ value increasedas the planar ratio increased. Of particular note is that the κ valuerose sharply to increase decomposition capability when the planar ratiorose to 2 and higher.

1. A method for producing a decomposer of organic halogenated compoundscomprising: a) providing an iron powder and a copper sulfate powder; b)mechanically mixing the iron powder and the copper sulfate powder in aball mill and in a dry method to plastically deform iron particles ofthe iron powder to give them a flat shape, to expose an inner ironmatrix of the iron particles, and to join surfaces of the iron particlesto surfaces of copper sulfate particles to produce the decomposer oforganic halogenated compounds, wherein the iron particles of thedecomposer have a flat shape with a planar ratio of 2 or greater.
 2. Themethod for producing a decomposer of an organic halogenated compoundaccording to claim 1, wherein the iron powder is reduced iron powder andthe copper sulfate powder has been obtained by dehydration treatment.